1. Technical Field
The present invention relates to a technique where an image is printed by forming dots on a printing medium.
2. Background Technology
Printing apparatuses which print images by forming dots on a printing medium are widespread. In such printing apparatuses, the dots are formed on the printing medium and the images are printed by discharging ink from a head at an appropriate timing while reciprocating the head above the printing medium. The gradation expression of the image is obtained by changing the number of white and black dots over an area with a certain fixed area set as a unit. A systematic dither method is known as such a method. In the systematic dither method, for example, in a case where 17 gradations are expressed, 4 pixels×4 pixels are set as one unit and it is determined whether dots are formed for each of the pixels by using a dither mask where 16 thresholds from 1 to 16 are arranged and comparing the image signal and the thresholds.
In a blue noise mask which is a typical example of a high image quality dot distribution dither mask, the thresholds are arranged such that the image has a blue noise characteristic when the image is binarized using the dither mask. This is because the dots become less conspicuous due to peaks of the spatial frequency characteristics of the image being set to come in a region with high spatial frequency where human visual sensitivity is reduced.
Japanese Laid-open Patent Publication No. 2007-15359 (Patent Document 1) is an example of the related art.